Greek police say two gunmen who hijacked a bus on the outskirts of Athens are demanding to be flown out of the country.

Authorities say there are more than 20 passengers being held hostage on the bus, which was seized early Wednesday en route from Marathon, east of Athens, to the city center. The bus driver and the ticket collector managed to escape along with one passenger.

The bus has been surrounded by police and there have been reports of shots being fired.

The gunmen have been described as foreigners, with some officials calling them Russians. Police say the hijackers have asked to be taken to the Athens airport.

There are no reports of injuries. The motive for the attack remains unclear.